Posts tagged "x86"
Show Your Working: Fixing a pile of bugs in MAME
Sun 03 December 2017
Show Your Working is a brand new segment where I write up things I have attempted to fix in open source software. Sometimes it's interesting to debug a problem yourself from first principles, even if the codebase is huge and you don't know anything going in. I will try and explain my thought process as I venture out into the weeds armed only with a butter knife.
If you haven't heard of it, MAME is an emulation project with the aim of supporting basically every arcade machine ever made. To do this, thousands of circuit boards and custom chips have been probed, analysed, decapped with acid, and ultimately written out as code. By adding board ROMs (whole dumps of the memory chips containing the copyrighted program data), MAME can emulate a system by connecting together drivers for all of the chipsets as per the layout of the original board. Recently, the sister project MESS has been merged back into the MAME main codebase, adding ~2200 consoles and home computer systems to the lineup. It is an incredible feat of volunteer-driven engineering.
Unlike other emulators, MAME places accuracy and preservation above all else. All CPU code is interpreted (with some IL speedups via the UML architecture) and runs time-shared in a single thread. Instead of tightly coupling the hardware code with the framework for performance, MAME has an infinitely-rewirable generic module architecture, to encourage reuse of chip drivers across platforms. Imagine MAME as a big box of all the different types of chips, plus some templates that explain what chips to take from the box and how to wire them together to make a system.
Also I was not exaggerating when I said every arcade machine ever made. The MAME source tree has slightly north of 6900 .cpp files, and a princely 2.9 million lines of code (4 million if you count headers!). This is a Big Codebase.